Currently, 3rd generation cellular communication systems are being installed to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. This is in contrast to time division multiple access (TDMA) systems, where user separation is achieved by assigning different time slots to different users.
In TDD systems, the same carrier frequency is used for both uplink transmissions, i.e. transmissions from the mobile wireless communication unit (often referred to as wireless subscriber communication unit) to the communication infrastructure via a wireless serving base station and downlink transmissions, i.e. transmissions from the communication infrastructure to the mobile wireless communication unit via a serving base station. In TDD, the carrier frequency is subdivided in the time domain into a series of timeslots. The single carrier frequency is assigned to uplink transmissions during some timeslots and to downlink transmissions during other timeslots. In FDD systems, a pair of separated carrier frequencies is used for respective uplink and downlink transmissions. An example of communication systems using these principles is the Universal Mobile Telecommunication System (UMTS™). Further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of UMTS™, can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a conventional cellular system, cells in close proximity to each other are allocated non-overlapping transmission resources. For example, in a CDMA network, cells within close proximity to each other are allocated distinct spreading codes (to be used in both the uplink direction and the downlink direction). This may be achieved by, for example, by employing the same spreading codes at each cell, but a different cell specific scrambling code. The combination of these leads to effectively distinct spreading codes at each cell.
FIG. 1 illustrates a known WCDMA (FDD) HSDPA radio framing/timing structure 100. The HSDPA radio framing/timing structure 100 has a 10 msec radio frame 105 comprising five*2 msec sub-frame periods. Each of the five*2 msec sub-frame periods is used to carry unicast data 110 and unicast control information on unicast control channels 115. As such, a single (unicast) scrambling code is used for the transmission of unicast data and control on the same physical resource.
In order to provide enhanced communication services, the 3rd generation cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service is multimedia services. The demand for multimedia services that can be received via mobile phones and other handheld devices is set to grow rapidly over the next few years. Multimedia services, due to the nature of the data content that is to be communicated, require a high bandwidth.
Typically, a wireless subscriber unit is ‘connected’ to one wireless serving communication unit, i.e. one cell. Other cells in the network typically generate interfering signals to the wireless subscriber unit of interest. Due to the presence of these interfering signals a degradation of the maximum achievable data rate, which can be maintained to the wireless subscriber unit, is typical.
The typical and most cost-effective approach in the provision of multimedia services is to ‘broadcast’ the multimedia signals, as opposed to sending the multimedia signals in an unicast (i.e. point-to-point) manner. Typically, tens of channels carrying say, news, movies, sports, etc., may be broadcast simultaneously over a communication network.
As radio spectrum is at a premium, spectrally efficient transmission techniques are required in order to provide users with as many broadcast services as possible, thereby providing mobile phone users (subscribers) with the widest choice of services. It is known that broadcast services may be carried over cellular networks, in a similar manner to conventional terrestrial Television/Radio transmissions.
Technologies for delivering multimedia broadcast services over cellular systems, such as the Mobile Broadcast and Multicast Service (MBMS) for UMTS™, have been developed over the past few years. In these broadcast cellular systems, the same broadcast signal is transmitted over non-overlapping physical resources on adjacent cells within a conventional cellular system. Consequently, at the wireless subscriber unit, the receiver must be able to detect the broadcast signal from the cell it is connected to. Notably, this detection needs to be made in the presence of additional, potentially interfering broadcast signals, transmitted on the non-overlapping physical resources of adjacent cells.
To improve spectral efficiency, broadcast solutions have also been developed for cellular systems in which the same broadcast signal is transmitted by multiple cells but using the same (i.e. overlapping) physical resources. In these systems, cells do not cause interference to each other as the transmissions are arranged to be substantially time-coincident, and hence capacity is improved for broadcast services. Such systems are sometimes referred to as ‘Single Frequency Networks’, or ‘SFNs’. In SFN systems, a common cell identifier (ID) is used to indicate those (common) cells that are to broadcast the same content at the same time. In the context of the present description, the term “common cell identifier” encompasses any mechanism for specifying SFN operation, which may in some examples encompass use of, say, a single scrambling code.
Broadcast solutions that are based on WCDMA MBMS technology tend to use long spreading codes and are associated with long transmission times per service or data block or even continuous transmission. This is a sub-optimal approach from a user device perspective, since the receiver needs to be in an ‘ON’ state for a large fraction of time, or even always in an ‘ON’ state. This can have detrimental impact in terms of viewing times for Mobile TV and other broadcast related services. The long or continuous transmission times per service demand that multiplexing of multiple services on the same carrier must be performed in the code domain.
In the field of broadcast communication, integrated mobile broadcast (IMB) is a standardised part of the 3GPP™ Release 8, and is ideally suited to address a growing market need for a global broadcast solution for mobile devices. IMB harmonises elements of existing 3GPP™ Release 7 WCDMA and TD-CDMA standards to achieve a unified solution incorporating the best technologies from both. IMB utilises a dedicated frequency carrier to support mobile broadcast and is an overlay network operating on a separate carrier to unicast traffic. Thus, IMB allows simultaneous operation of both broadcast and unicast traffic on two separate carriers. IMB can also be used in standalone operation, i.e. without an associated unicast carrier.
FIG. 2 illustrates a known IMB radio framing/timing structure 200. The IMB radio framing/timing structure 200 also has a 10 msec radio frame 205 comprising five*2 msec sub-frame periods. Each of the five*2 msec sub-frame periods is used to carry broadcast data 210 and broadcast control information on broadcast control channels 215. As such, a single (broadcast) scrambling code (or common cell identifier) is used for the transmission of broadcast data and control on the same physical resource.
For mobile operators that have unpaired spectrum that can be dedicated to broadcast transmissions, IMB provides an excellent solution, since all of the radio frame can be configured for broadcast transmissions. For mobile operators that only have paired unicast spectrum and do not have unpaired spectrum, deployment of IMB services is not possible. One option for these operators is to multiplex the IMB waveform with the unicast transmissions. However, the notion of a Common cell identifier when combined with unicast data does not exist, since the unicast data needs to be unique, on a per cell basis.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting broadcast transmissions on a unicast carrier over a cellular network would be advantageous.